Falling film evaporators and especially evaporators of the plate type or free flow falling film type have proven its superiority in performance and energy consumption over other types of evaporators, such as, for example, Kestner LTV type evaporators, forced circulation type evaporators and tubular falling film evaporators. The advantage of the free flow falling film type evaporator over other types of evaporators, for instance, tubular type evaporators using the inner surfaces of a plurality of tubes for evaporation are many. For example, in the plate type falling film evaporators the evaporation surface is exposed to the surrounding vapor space along most or all of the vertical surface thus leaving a large free area between the heating elements for release of the vapor generated thereon. The equivalent vapor release area in a tubular type evaporator would be the horizontal cross-sectional area of a vertical tube. This means that in the tubular type evaporator a suspension of liquid and vapor exits from the tube at relatively high velocity and which suspension must be separated in the same manner as in a Kestner type evaporator utilizing a large receiving space located below the tube bundle. In the free flow falling film type evaporator the vapor simply escapes from between the plate elements and flows freely into the space defined by the circular vessel surrounding or housing the square or rectangular heating element package. The vapor generated will travel in a curved pattern, i.e. initially in a slightly downward direction between the plate elements due to the inertia of the liquid flow and when approaching the edge the vapor will travel in a slightly upward direction and thereafter in a substantially vertical upward movement when flowing out into the free space between the element package and the vessel wall. The vapor raises and accelerates on its way up and reaches its maximum velocity when passing over the upper edge in the upper part of the heating elements. During this upward movement, the vapor can carry along and entrain some of the liquid which has just been deposited on the heating elements. Normally, the vapor is thereafter collected in a chevron type droplet separator and thus permitted to flow out through the top of the vessel. Such apparatus are, for example, used to a considerable extent in the pulp industry.
For reasons of economy, for example, in the recovery of chemicals, and for reasons of environmental preservation, care must be taken that the droplets of liquid are not carried along by the vapor to outside of the evaporator which has been a problem with the known evaporation systems.